Which MOSFET / IGBT can afford 5A current normal operation and 40A current at 1-2 seconds?

I am considering to use a transister to be a switch for over current cut-off. It is controlling a 220AC(rectified)DC motor and monitoring it go over current or not. At normal operation, motor current is 5A. However, if it is over current, it maybe rise to 40A at 1-2 seconds and transistor will be cut-off.

Which MOSFET / IGBT can afford 5A current normal operation and 40A current at 1-2 seconds and VDS need > 400V ? Of course, price is one of the most important parameter for me. But I think functionable is the most important part.

Reply to
Electronic Swear
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There are many candidates for a job like that, but if you can get a pair in a SOT-227B power case, with insulated heat-sink plates, that would be good. Fairchild's 600V 40 IGBT, HGT1N40N60A4D, is a good example,

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It's actually a bit better than you need. It will drop 1.4V at 5A, rising to 2.1V at 40A, disspating only 84 of its rated 300 watts.

To make a AC switch you'll need to drive two such IGBTs back-to-back (with sources and gates connected) using a floating drive circuit.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Greetings.

There are many ways to skin this cat. If you can tolerate a "live" heatsink (or in this case more of a heatspreader), then I think I might suggest paralleling three or perhaps four IRFP460P MOSFETs:

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For some reason these are surprisingly cheap and offer quite decent bang for the buck (at least compared to a number of other Digikey offerings). They are TO-247 parts rated at 500V, 0.27 Ohm on resistance, and 280W power dissipation. The novelty of this solution (compared to IGBT solutions) is that the normal power dissipation at 5A will be quite minimal. Of course, you will still need a nice heatsink/heatspreader with minimal thermal resistance between the TO-247 case and heatsink to safely handle the 40A fault current.

Reply to
Fritz Schlunder

Thank you for your suggestion.

Basically, I have tried to use IRFP450 before. Of course there is a heatsink behind the body of the transistor. However, it is also failure in that operation. At 40A current 1-2 seconds, the transistor cannot switch to cut-off. Then the transistor finally burnt out with short the junction.

If I use for the IRFP460, what size of heatsink should I need for? Topically, the IRFP460 is not much affordable a higher current then IRFP450. So, is it also afford that high current at 40A?

Reply to
Electronic Swear

Of course the usual answer, and not a bad one, is to use a mechanical switch, such as a contactor. But if you must use an electronic switch for some reason, you might consider the parallel combination of a FET and a high-current IGBT (but perhaps a smaller one than I suggested). This can give you the best of both worlds. (At higher AC currents one can consider using a large GTO (gate-turn-off) thyristor.)

As you know, the IRF450 is a 0.4-ohm FET, and the IRF460 is a 0.27-ohm part. So a '450 will heat 50% more than a '460, which is significant.

Back to a '460, we know its 0.27-ohm Ron will be increased by 2.3x for a hot die temp. Consider P=I^2R for 40A with 0.27*2.3 ohms = 933 watts. Even though we have some thermal mass working in our favor, we can see that a '460 FET, through an insulator to a cool heatsink, can still get us into serious trouble during a several-seconds long event. (A quick examination of the Effective Transient Thermal Impedance plot shows the benefit of the FET die's thermal mass is gone well before two seconds.)

--
 Thanks,
    - Win
Reply to
Winfield Hill

You are right. Initially, I want to use relay as a switch. When the current up to 40A, my control circuit try to switch-off the relay, however, it cannot be switch-off even there are a control signal to trigger.

The problem is if I want to control a relay the switch-off from 40A current, the relay cannot be switch-off because of the high attractive force by high current.

I don't know why there are such kind of problems. I use a

10A relay, but the problem still happen. When I change to use a very large size AC relay, then it can trip-off. It is not possible for me to choose large side component for built-in my product.

Reply to
Electronic Swear

[snip]

Yes, the more usual solution would be a motor-rated AC circuit breaker upstream of the rectifying bridge.

See Farnell 717-5670, at GBP 9 pounds.

This is an ABB circuit breaker which will carry 6Arms permanently without tripping, and will allow 48-72Arms to flow for about one second before tripping.

For a single phase rectifier without capacitive smoothing divide those AC currents by 1.1 to get the approx DC. So that would be 5.5Adc carry-current, tripping after one second at 44 to 65Adc.

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Tony Williams.
Reply to
Tony Williams

A single IRFP450 device will not be adequate, regardless of the size of heatsink. Although better, a single IRFP460 will also be inadequate. You will likely need to use at least three IRFP460 devices in parallel all mounted to a single or three separate heat sinks. I'll provide the link to the IRFP460P datasheet again for reference:

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We notice from figure 11 in the datasheet (transient thermal impedance curves) that for a two second event we will receive no benefit from the thermal inertia of the IRFP460 die. Therefore we will need to parallel enough IRFP460 devices such that they could handle 40A of current indefinitely (provided they had enough heatsinking).

So lets assume we use three IRFP460P devices in parallel for your solution. How big should the heat sink be?

First find the steady state dissipation at 5A. I^2 * R at 25 deg. C with three devices in parallel will be around 2.25 Watts. Each of the three devices would therefore be dissipating around 750mW of heat. Giving this is a fairly small number, and we will need a heatsink anyway (for it's thermal inertia), we will make the approximate assumption that the MOSFET dies will be operating at say 50 deg. C prior to an overcurrent event. This is a fairly arbitrary assumption, but it gives us a starting point to work with.

We will therefore also assume the heatsink temperature is around 50 deg. C prior to an overcurrent event. Since the ultimate objective of these calculations it to insure a design that will keep the MOSFET die temperatures at or below 150 deg. C (their rated maximum), we will need to know the thermal resistance of the MOSFET dies to heat sink. From the IRFP460P datasheet the thermal resistance should be (without electrically isolating thermal pad, but with a flat heatsink that has been greased and appropriately torqued) around 0.45 + 0.24 deg. C/W = ~0.7 deg. C/W.

So now how much heat must the MOSFETs dissipate at the end of the 40A overcurrent event? Let us assume we allow them to reach the maximum die temperature allowed (150 deg. C) after two seconds. This means their on resistance will be around 0.27*2.5 (from figure 4) = 0.675 Ohms. For three in parallel the total MOSFET set resistance is 0.225 Ohms. So I^2 * R at

40A and 150 deg. C is 360W. Each device must dissipate one third of this or 120W.

Now we can approximate the thermal rise due to the thermal resistance between die and heatsink. 120W * (0.7 deg. C/W) = 84 deg. C. So 150 deg. C - 84 deg. C = 66 deg. C. Since we assumed the heatsink would start at a temperature of 50 deg. C (mainly due to ambient heat), this means we have

66 - 50 = 16 deg. C heatsink temperature rise to work with. So we need to select a heatsink with enough thermal mass so as not to rise more than 16 deg. C given our anticipated total energy lost during the overcurrent event.

During the overcurrent event we assumed the MOSFET set is dissipating 360W (given a die temperature of 150 deg. C, which is probably fairly accurate since the MOSFET dies will heat up to near 150 deg. C very quickly compared to the two second event since most of the thermal rise will occur between die and heatsink). Since the overcurrent event can last up to two seconds, the total energy dissipated should be no more than about 360 W * 2 s = 720 Joules. So we need a heat sink capable of taking 720 Joules with only a 16 deg. C temperature rise.

Assume we select aluminum as our heatsink material. Aluminum has a specific heat of 900 J/(kg*K). Solving for the heat sink mass (720 J) * (kg*K/900 J)

  • (1/16 K) = 0.05 kg. Since aluminum has a density of 2700 kg/m^3, we need a heatsink that has a volume of at least 18.5 cm^3 and mass of 0.05kg. You could use either one heatsink of 18.5 cm^3 or three heatsinks of 6.2 cm^3. The optimal heatsink shape in this case would not have large fins, but be one large rectangular block, since a solid block would have less thermal resistance between the TO-247 contact point and the rest of its thermal mass.

The heat sink/spreader does not need to dissipate much steady state heat, so you could pot the whole heatsink or cover it with something electrically insulating if desired to increase technician safety (and/or comply with possible safety organization specifications). Unfortunately when using only three IRFP460P devices you will not be able to use electrically isolating thermal pads (unless they are extremely good) since they will increase the thermal resistance from package to heatsink too much. SOT-227 packaged devices are usually extremely expensive, but it does have the very novel feature that you get an electrically isolated heat sink tab.

So does this help answer your questions?

Reply to
Fritz Schlunder

Hmm... The TO-247 package has a mass around 6 grams. If we assume the package has a specific heat around that of copper of 385 J/(kg*K), then by my calculations you should be able to safely handle the overcurrent condition if you simply parallel five or more IRFP460P devices with no heat sink necessary. Someone want to double check me?

I recommend you use this strategy. Digikey sells the IRFP460P device in ten unit quantities for US $1.386 each. So a full solution of five IRFP460P devices with no heat sinks (or heat sink grease, screws, safety issues of "live" heatsink, mounting torque reliability problems, etc.) would only cost $6.93 if they were purchased in ten unit quantities from Digikey.

Oh yeah, make sure to read this first:

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Try to keep the parasitic components well balanced across all devices, and use separate gate drive resistors.

Reply to
Fritz Schlunder

If I go to use 3-5 pieces of IRFP460, I cannot afford that high costing as I just want to complete it within US$1-2.

Is there any cheaper method to do the same job? Of course, there are some IGBTs can handle high current rating. However, the cost is still very expensive.

May I know that any mechanical device can do as a switch to avoid over current?

Reply to
Electronic Swear

Is this is a mass-production design? Thousand-piece quantities or higher? How much are you paying for the motor?

I may have misled you by mentioning Fairchild's very capable HGT1N40N60A4D 600V 40 IGBT. Smaller low-cost parts can handle 40A for a few seconds, with a suitable heatsink. For example a Fairchild HGTG20N60B3 costs about $2 each qty 1k. It drops under 1.5V at 5A, dissipating less than 7.5 watts.

You start by keeping the case temp down with a moderate-sized heat sink. This allows the IGBT junction to increase by 75 to 125C during a short 40A event, as the heat spreads to the tab and some adjacent aluminum. The HGTG20N60 will drop about 2.3V at 40A at the beginning (junction at 40C), which is 92 watts, increasing to about 2.8V (112 watts) as the junction heats up. This is well within the rating of the IGBT (165 watts 25C case) if you directly bolt on a slab of aluminum with a small amount of grease but _without_ an insulator, before some insulation to the rest of your finned heatsink.

I assume you're making the on/off signals? You'll have to be sure to shutoff the IGBT after only 2 to 3 seconds at 40A.

Tell us some detail about what you're working on.

If you're rectifying 220 AC for your DC motor, you can use two triacs in the bridge, in place of diodes, each conducting 5A (or 40A) for every other half cycle. Low-cost TO-220 commodity 25A triacs can conservatively handle the task with a modest heatsink. You'll need gate triggers each half cycle, or a continuous drive.

For example, Philips' BTA225-600B costs $0.75 qty 500 at Future, ST's BTB08-600B is $1.15 qty 1.5k at Future, Teccor Littlefuse's Q6025L6 is $1.69 qty 1k at DigiKey. Some types have insulated tabs, but you'd have to examine their thermal-resistance ratings.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Thank you for your opinion.

Basically, I am doing over current protection when the motor is going to lock rotor. I am not monitoring the current but monitoring the speed of the motor. I use the power Mosfet /IGBT to be a switch for cut-off.

Of course, in normal operation, motor is running and the transistor need to handle 5-6A current. However, if there are rotor locking, the current will rise to 40A. When circuit detect the speed is zero, it will trigger the transistor to cut-off.

I am quite interested on using triac to cut-off. However, I still not catch your point on where to place the triac on the bridge recifier side. Can you give me a more clear schematic on it?

Thanks~

Reply to
Electronic Swear

Any answers for us here?

[Well, I guess you can see how top-posting messes up a conversation.]

I haven't used this configuration myself, but I've seen others do it. Here's the basic idea:

.. H ----------------, .. SCR1 | .. ,--A-|>|-K--o--o----|>|---, .. | G | D1 | .. | '--xx--' | .. N ----| -------, | .. | SCR2 | D2 | .. o---|>|--o-----o----|>|---+--o-----------, .. | | | snubber | .. | '--xx--' _|_ cap DC + .. | SCR gate --- MOTOR .. | transformers | | - .. '----------------------------o-----------'

You are using unfiltered rectified AC for your DC motor, right? If you want to use this scheme with large DC filter capacitors, series inductance must be added. In any event, snubber caps are needed.

I was mistaken, SCRs are used instead of triacs. This saves a few pennies, an MCR25M costs $1.31 qty 10 at DigiKey, and $0.71 qty 1k.

If you use delayed-phase switching, you can control the motor power.

You can also switch on the high side of the dc output line,

.. AC line _____ .. H --------------, G-------| | .. | |/ | direct .. ,---|>|---o---A-|>|-K--, | gate .. | D1 SCR1 | | control .. | ,--|----|_____| .. | D2 |/ | | .. o---|>|---o-----|>|----o---o---o----, .. | | SCR2 _|_ | + .. N ----|---------' --- MOTOR .. | snubber | | - .. '--------------------------o--------'

Either form is similar using gate trigger transformers, but high-side switching allows you to use convenient direct gate-drive connections. However, this requires the control circuits to ride on the high-side of the switched voltage, which can be VERY DANGEROUS during testing.

In testing direct ac control it's required that you NEVER connect your scope ground to the logic ground, etc., and you must take exceptional safety precautions. It's required for Hot switching, but if you use a form of Neutral line switching, you should still absolutely _not_ count on the ac line being properly wired.

Perhaps others who have more experience with 1 HP motors will comment further about SCR motor control.

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 Thanks,
    - Win
Reply to
Winfield Hill

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 Thanks,
    - Win
Reply to
Winfield Hill

This is correct, because allowing the condition to exist for several seconds forces one to use 40 to 50A parts, or a pair of 25A parts. On the other hand, if the over-current condition was limited to say 6 cycles, or 0.1 sec, a single low-cost 10 to 12A IGBT could be used. The savings would more than compensate for the added power resistor.

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 Thanks,
    - Win
Reply to
Winfield Hill

You could save money using lower-cost 8A jelly-bean SCRs, if you can limit the overcurrent time to 0.1s or less, using a sense resistor. You'd also benefit from a reduced heat-sink thermal-mass requirement.

.. AC line _________ .. H --------------, G------| | .. | |/ | direct | with overcurrent .. ,---|>|---o---A-|>|-K--, | gate | sense resistor .. | D1 SCR1 | | control | for stalled motor .. | ,--|---|_________| shutdown .. | D2 |/ | | | .. o---|>|---o-----|>|----o-----o-//-o--o--------, .. | | SCR2 sense _|_ | + .. N ----|---------' --- MOTOR .. | snubber | | - .. '-------------------------------------o--------'

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 Thanks,
    - Win
Reply to
Winfield Hill

I think this is not very reliable, because you assume the motor will be running at a certain speed in a few seconds after switching it on. Why not use a small series resistor and sense the current? If it goes over a specific current, you shut the circuit down or turn the transistor off for 1 AC cycle. The latter also provides a crude soft-start mechanism. Also, you might be able to get away with 1 FET or IGBT because you can determine the current handling limits of the transistor more precisely.

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Reply to
Nico Coesel

The DC motor is operated at (rectified) 220Vac. It is for food juicer. We don't want to use current sensing because the motor start-up current is very large. Similiar to what the current at lock rotor. As a result, current monitoring is not a very good method. Even the motor will be over-loading for a short moment when the user is putting a very hard / big food for juicing at the beginning. The current will be very large at the beginning but will drop to a suitable value. However, we cannot cut-off the motor source because of over-current. We will allow the motor operating at a very high current for a short time, several seconds.

The motor is for masss production and look for safety protection as well. If use fuse on protection, it is very inconvenient for exchange. We want some active protection rather than passive components.

I will try for using the SCRs at the bridge rectifying. However, the current rating of the SCR just 25A is enough or not? And any detail on snubber capacitor?

Reply to
Electronic Swear

Perhaps it isn't at 40A for the entire 2 seconds... Or... Hmm, if the machine is straining, wihch the user can tell, I think 2 seconds is a rather long time... Try it out.

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 Thanks,
    - Win
Reply to
Winfield Hill

Why not use current sensing and let the processor decide when the special case of a startup means forgiving high current a little longer?

Another case the microprocessor can detect.

Each SCR works at 50% duty cycle. Study the datasheets.

It's past time for you to start reading the links we've provided and learning about the scene.

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 Thanks,
    - Win
Reply to
Winfield Hill

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